Metal-sheet printing machines and metal-sheet painting machines are used for printing and painting sheets of metal, from which cans for packing purposes, lids for these cans or for glasses, closures for bottles or similar articles made of thin sheet are produced. These sheets of metal are referred to as metal sheets below. The printing machines and painting machines are also used for printing and/or painting sheets of other materials, for example of plastics material.
In these machines a number of printing cylinders and transfer drums follow in succession. In the case of many machines the axes of these cylinders and drums are situated approximately at the same level. In the case of other machines the axes of the transfer drums are situated at a lower level than those of the printing cylinders.
The printing cylinders have a substantially closed cylindrical surface. The transfer drums, on the other hand, preferably comprise a tubular drum core of smaller diameter on which are arranged some components which rotate with the drum core. The outermost cylindrical path of movement of these components defines an imaginary cylindrical generated face of the transfer drums, to which part of the remarks set out hereinafter relates.
In the case of both the printing cylinders and the transfer drums, grippers which grip the front edge of the metal sheets to be printed or painted and which entrain them are arranged at a specified peripheral point. Grippers of this type are not provided for the rear edge of the metal sheets.
During their passage through the printing machine the metal sheets are transferred from the printing cylinder to the transfer drum and from the latter to the printing cylinder. In this case they are gripped at their front edge portion by the associated grippers and are passed on from one transfer station to the next one. During this the metal sheets change their curvature.
During the passage with the printing cylinder, which should in fact be referred to as a counter-printing cylinder, the metal sheets rest with their rear side on the printing cylinder. In this case the metal sheets are guided by being guided through the narrow gap between the printing cylinder and a blanket cylinder which transfers the print image with a specified coloration onto the front side—facing it—of the metal sheets.
During the passage with the transfer drum the metal sheets are guided on their rear side by guide strips which are arranged at a certain distance from the imaginary generated face of the transfer drum. These guide strips extend over a peripheral region of the transfer drum which is smaller than the peripheral region between the first and the second transfer station of the transfer drum.
During the transfer of the metal sheets from the transfer drum to the immediately following printer cylinder, after a specified distance, of travel, of the metal sheets their rear longitudinal portion no longer rests against the guide strips on the one hand and does not yet rest against the printing cylinder on the other hand. This leads to the metal sheets becoming unstable and tending to vibrate outside their clamped front edge area under the action of internal and external forces, such as for example elasticity and forces of inertia. These phenomena occur even in the case of printing machines, the axes of which are situated at the same level.
In order at least to reduce these vibrations it is known for guide elements to be applied to the transfer drum, the guide elements being arranged in the end region of the metal sheet in such a way that the rear edge of the metal sheet can rest against them. For this purpose these guide elements have an abutment face which is at a specific fixed angle of inclination with respect to the tangent of the generated face of the transfer drum.
Since the guide element moves away from the metal sheet on its circular path of movement, the angle of inclination between its abutment face and the rear end potion of the metal sheets changes. During the transfer of the metal sheets from the transfer drum to the immediately following printing cylinder the vibrations occurring in the metal sheet and the instability of its rear portion therefore have the result that not only the end edge of the metal sheets but, also, their entire end portion and with it the end portion of the printed area rest against the abutment face of the guide element, and as a result the print image is blurred.
In the case of printing machines with a vertical offset of the axes of the printing cylinders and the transfer drums, this error can occur to an increased degree. In certain cases this blurring of the print image makes the metal sheets unusable.